Velocity wet deposition

Assume the dry deposition velocity over a New England forest for particles emanating from a midwestern U.S. power plant averages 0.5 cm/sec on a certain day. The scavenging coefficient in a rain-forming cloud over the forest is 10 3/sec. If the cloud is 500 m in vertical extent, what is the equivalent wet deposition velocity, Vw, for rainout of the particles ... 

In this situation, the wet deposition velocity is 100 times faster than the dry... 

Note that according to the definitions of (20.7) and (20.8) the relation between the wet deposition velocity uw and the washout ratio wr is... 

Wegener-Bergeron-Fmdeisen mechnmsm, 831 Weibiill disti ibiition, 1268 moments, 1271-1272 Wet deposition velocity, 1002 Wind speed ... 

S A1 Dry deposition velocity for gases and chemical vapors from air-to-the soil and ground surfaces SA2 Dry deposition velocity for particles and associated sorbed chemicals from air to earthen surfaces SA3 Wet deposition velocity for gases and chemical vapors from air to earthen surfaces S A4 Wet deposition velocity for particles and associated sorbed chemicals from air to earthen surfaces SA5 Air-side MTC for gases and vapor from soil and ground surfaces into the lower atmospheric boundary layer... 

For wet deposition, it is assumed that the rain scavenges Q (the scavenging ratio) or about 200,000 times its volume of air. Using a particle concentration (volume fraction) vQ of 2 x 10 n, this corresponds to the removal of Qvq or 4 x 1CF6 volumes of aerosol per volume of rain. The total rate of particle removal by wet deposition is then QvqUrAw m3/h, thus the wet transport velocity QvqUr is 4 x 1(T10 m/h. 

Inputs and outputs to the lake have been measured to calculate net retention for the pre-acidified lake. Precipitation inputs of sulfate were based on data from wet collectors (1980-1983) compiled by the National Atmospheric Deposition Program (NADP). SO2 inputs were calculated from regional ambient air concentrations (22) usinga deposition velocity of 0.5 cm/sec. Aerosol sulfate was estimated from NADP dry bucket measurements and from dry bucket and snow core measurements made in this study (22). Groundwater inputs occur largely at the southeast corner of the lake and were calculated from modeled inseepage (21) and measured sulfate concentrations in a well located in the major inseepage area. Sulfate output was estimated from mean lakewater sulfate concentration and modeled outflows. 

For example, taking suitable values of deposition velocity, listed for example in Table II, and data from the SURE (2), estimates of the annual average dry deposition rate for sulfur are the order of 6-60 kgS/ha-yr in the East. This is compared with values of 4-16 kgS/ha-yr in wet deposition. Although dry deposition levels of NO ... 

Several integral measurement methods were evaluated in the early stages of this program. The wet-chemistry bubbler system deployed in Europe for remote measurement purposes cannot distinguish different chemical species. Since the deposition velocity is very species dependent, clear distinction among different chemical species is required to derive dry deposition rates. Filterpack methods have limitations as well. At the 1982 Technical Committee meeting of the National Atmospheric Deposition Program, conducted in St. Louis, a... 

Dry deposition is parameterized via a resistance approach in which resistances depend on particle size and density, land-use classification and atmospheric stability (Wesely 1989 Zanetti 1990). Wet deposition is included via below cloud scavenging (washout), using a parameterization based on precipitation rates (Baklanov and Sprensen 2001) and scavenging by snow is parameterized using the scheme by Maryon and Ryall (1996). The terminal settling velocity is considered in both the laminar case, in which Stake s law is used and the mrbulent case in which a iterative procedure is employed (Naslund and Thaning 1991). For very small particles a correction for non-continuum effects is used. 

Values are means standard errors for 2 years of data. Numbers of observations range from 15 (HNO3) to 26 (particles) to 128 (precipitation) to 730(802). In comparing these deposition rates it must be recalled that any such estimates are subject to considerable uncertainty. The standard errors given provide only a measure of uncertainty in the calculated sample means relative to the population means hence additional uncertainties in analytical results, hydrologic measurements, scaling factors, and deposition velocities must be included. The overall uncertainty for wet deposition fluxes is about 20% and that for dry deposition fluxes is approximately 50% for SOj", Ca ", K", and approximately 75% for NOj" and... 

The variable emission rates and the meteorological parameters — wind velocity (dilution factor), wind direction (contribution of other sources) and the occurrence of atmospheric wet deposition - are the main parameters responsible for the variation in ambient air concentration levels with a factor of 10. 

Cadle and Groblicki (J[0) determined the composition of dew deposited naturally on glass. Teflon, and plastic surfaces in Warren, MI. Dew composition was compared to wet and dry deposition obtained the previous year at the same site. In this paper, the comparison of dew and rain composition is updated and the results of a new study of the composition of artificially-generated dew are reported. Deposition velocities to the dew of SO2, HNO and K are also presented. 

In this work the intent was to perform a more comprehensive analysis of the dew and to compare deposition velocities to a wet and a dry surface. In order to bring more control to the experiment, dew was generated artificially by attaching cooling coils to the Teflon covered copper plate. The dry plate counterpart consisted of a 1 m glass collector covered with Teflon and mounted in the same manner as the copper plate. All deposition rates are based on the actual area of the plate rather than the projected horizontal area, which was... 

Analysis of Environmental Data. Although the methodology for analyzing the data has been previously reported ( ) there are some differences that should be noted. First, a later version of the RAPS data base was used as an initial sourse. Second, time-of-wetness in this paper is defined differently, thus, a relationship to calculate relative humidity from temperature and dew point is based on data for dew points greater than 0 C. Third, deposition velocities are calculated from boundary layer theory rather than empirical relationships. 

Pollutant Fluxes. Hourly deposition velocities were multiplied by hourly pollutant concentrations to get hourly pollutant fluxes. These were summed over exposure periods for hours of wetness with different critical relative humidity criteria (75 to 90% in 5% intervals). Average fluxes were then calculated by dividing by the time-of-wetness. The results were compared with fluxes calculated by multiplying average deposition velocities for a period by the average pollutant concentration during times of wetness. The values by the two methods were fairly consistent. 

Fluxes of TSP were calculated by multiplying average TSP by two-tenths of the average deposition velocity for gases (actual deposition velocities vary considerably with the size distribution of particles). Rain fluxes were calculated by dividing the amount of rain for an exposure period by time of wetness. 

Reference (3) reanalyzed data from a number of such outdoor test sites and derived an SO2 deposition velocity of 1.55 - 1.75 cm./sec. for zinc, operable only during times of surface wetness. In that report, reference was made to SO2 deposition velocities over water surfaces (1.6 cm/sec), which are likely to be low turbulence situations. It was also noted that the deposition velocity to water is dependent upon atmospheric stability. SO2 deposition velocities for copper and aluminum were less straightforward (3), but appeared to be somewhat lower, perhaps reflecting less chemically active surfaces. The zinc result is reasonably consistent with the theoretical values developed above from boundary layer theory and tests, since it lies between these values (1.2 - 3.7 cm/sec). Incorporating actual test site wind speeds could obviously help reduce the scatter in these determinations. 

We have shown the importance of local free stream velocity (uqo) in controlling deposition velocity. There may be a relationship between Uo and time-of-wetness, either through variations in synoptic conditions or through heat transfer and evaporation. Such an interaction could modify the estimation of an effective annual average SO2 deposition rate. As mentioned above, one must also account for surface orientation in predicting time-of-wetness. 

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